Heretofore, the lubricant base oil was produced by using crude oil as a raw material. Recently, it is demanded to improve performances of motor oil (lubricant oil for automobile) and hence the lubricant base oil is required to have a high viscosity index and a low pour point. Also, it is required to reduce an influence of the lubricant oil on an environment. For this end, it is necessary to reduce sulfur content, nitrogen content, aromatic content and so on in the lubricant base oil. However, in case of using the crude oil as a raw material, it is difficult to reduce these contents. Nowadays, there is got a lot of attention on lubricant oil produced by a hydroisomerization of a Fischer-Tropsch synthetic wax being mainly composed of an isoparaffin and having substantially no aromatic content, naphthenic content, olefinic content, sulfur content, nitrogen content and so on as a raw material.
When the lubricant base oil is produced by using a normal paraffin such as Fischer-Tropsch synthetic wax or the like as a raw material, a step of removing the normal paraffin from oil produced by the hydroisomerization, i.e. a dewaxing step is usually conducted in order to sufficiently assure a low-temperature fluidity of the lubricant base oil (JP-B-6-62960). Therefore, as the isomerization sufficiently proceeds and the content of isoparaffin becomes large, the yield of the base oil at the dewaxing step increases.